For metal sheet parts (such as 2000 series, 6000 series, 7000 series aluminum alloy sheets and 22MnB5 boron steel sheets) capable of being thermally treated and strengthened, it is usually necessary to thermally treat and strengthen the formed parts (for aluminum alloy sheets, solution hardening is performed first to form a supersaturated solid solution, and then artificial aging precipitation strengthening is performed; 22MnB5 boron steel sheets are first heated and thermally insulated for austenization, and then rapidly quenched to form martensite). However, since the metal sheet parts need to undergo multiple heating and cooling during the heat treatment, the shapes and dimensions of the parts are easily changed due to uneven thermal expansion and contraction, thermal stress and internal stress in this process. In order to prevent or control the shape and dimensions changes of the parts during the heat treatment strengthening process, it is often necessary to use a complicated constraining tool to limit the sheet parts, or to perform the correction after the heat treatment strengthening is completed. This additionally increased subsequent processing inevitably results in a complicated forming process, poor consistency in shape and dimensions accuracy of parts, and low forming efficiency. For parts with complex shapes, even if the above treatment is performed, the required shape and dimensions accuracy often cannot be achieved.
In order to avoid the adverse effect of the heat treatment process on the shape and dimensions accuracy of the parts, a method for achieving metal sheet forming and heat treatment by using an identical die is provided; that is, by using the identical die, the forming of metal sheet parts is achieved first to obtain required shapes and dimensions, and then heat treatment is performed under the conditions of keeping the die closed to obtain required strength properties. Hot stamping of boron steel (such as 22MnB5) and hot stamping of aluminum alloys (such as 6061 and 7075) belong to this type of forming technology. During hot stamping, both an upper die and a lower die used are at lower temperatures (below 150° C.). A metal slab heated to a certain temperature is quickly placed on the die and die assembly is quickly performed to complete part forming. After the part is formed, cooling water is directly introduced into the closed upper die and lower die to lower the temperature of the die and the rapid cooling of the part is implemented by using a cold die. In the forming process, since a hot slab is cooled very quickly after contact with the upper die and the lower die that are cold. In order to ensure that the metal slab is at a sufficiently high temperature during the forming process to ensure good forming properties, it is required to complete the forming process in a very short time (1-3 s), which raises a high demand on a forming device. More importantly, the cold die and the hot metal slab are in partial contact for most of the time. The temperature distribution of the metal slab at a certain moment in the forming process is not conducive to the smooth forming of the entire part, which easily results in local forming defects such as partial wrinkles and cracks. Therefore, it is difficult to use this method for forming a complex-shaped metal sheet part.
In order to prevent or reduce the adverse effect of the cold die on hot metal sheet forming, the invention patent (patent title: METHOD FOR FORMING ALUMINUM ALLOY METAL SHEET PART BY USING COLD AND HOT COMPOUND DIE, patent number: ZL201210124230.X) proposes a solution of compounding of a cold die and a hot die, that is, the temperature of a metal slab in the forming process is slowed down by using a hot lower die to smoothly complete the formation of the part, and then the rapid cooling of the formed hot part is implemented by using a cold upper die. The method can reduce the mutual influence of the hot forming and the cooling quenching process to some extent, but still requires the hot forming process to be completed at a faster speed, and also needs to take necessary measures to ensure that the formed part and the cold upper die are in good contact to achieve effective quenching. This greatly limits the application of this method.